AN INTEGRATED SEDIMENTOLOGICAL-GEOCHEMICAL-PALEONTOLOGICAL ANALYSIS AS A PROXY FOR CARBONIFEROUS CLIMATE CHANGE: THE ROLE OF FINE-GRAINED SEDIMENTARY ROCKS

Fine-grained sedimentation dominated deposition, from NW Europe to Eastern Canada, in the shallow epicontinental Carboniferous seaway. Using an integrated approach combining high-resolution sedimentary facies analysis, whole rock geochemistry, palynology and δ¹³C, it is possible to unravel long-term climate change records.

The predominant Pennsylvanian silt-bearing clay-rich mudstones have textures including: (1) Graded beds (700-2600μm thick), with subtle basal erosion surfaces, that fine up from silt-rich to clay-rich bioturbated mudstones (2.1-2.4% TOC). Original bedding is preserved even where bioturbation is more pervasive. (2) Structureless mudstones (1.7-3.4% TOC) that contain sparse silt-sized grains and agglutinated foraminifera. The δ¹³C values suggest terrestrial organic matter is dominant in these mudstones. These sediments reflect well-oxygenated bottom conditions during periods of lowered sea-level when clastic supply, via gravity flows and wave-enhanced mudflows, dominated sedimentation.

Grey to black clay-rich mudstones characterise ‘marine bands' that have a diverse pelagic fauna, high TOC values (3.0-6.3%) and abundances of certain trace elements (e.g. V, As, U, Mo) and distinctive organominerallic aggregate fabrics. Variants of this texture suggest more pervasive bioturbation. δ¹³C values suggest a dominance of marine amorphous organic matter, with a terrestrial component. These intervals reflect high productivity, with deposition from suspension, during periods of limited clastic supply when marine transgressions caused deltaic systems to retreat to basin margins. Bottom-waters were variably oxygenated but, despite elevated Mo concentrations, a euxinic water column was unlikely.

Elevated kaolinite abundances characterise marine band intervals, typically interpreted as reflecting warm interglacial climates with high humidity and rainfall that enhanced chemical weathering, however our analysis reveals that the kaolinite is of authigenic origin. Therefore systematic changes in sediment delivery mechanisms and water column productivity can be linked to glacioeustatic sea-level change but the use of the clay mineralogy as a climatic proxy and trace element geochemistry as a redox indicator is challenged.